A magnesia aggregate constituting part of a magnesia brick is generally excellent in erosion resistance against basic slags such as converter slag, but is poor in slag penetration resistance.
The poor slag penetration resistance of the magnesia aggregate is attributable to the following cause. The magnesia aggregate more specifically comprises a silicate matrix having a low melting point, and a plurality of particles of periclase (MgO) having a high melting point of 2,800.degree. C., which are dispersed throughout the silicate matrix. When molten slag comes into contact with the surface of the magnesia aggregate having the above-mentioned structure, the silicate matrix having the low melting point melts and molten slag penetrates into gaps between the particles of periclase which then float in the molten silicate matrix. This causes loosening of the surface layer structure of the magnesia aggregate, thus allowing molten slag to penetrate further deeply into the interior from the surface of the magnesia aggregate.
When molten slag penetrates into the surface layer of the magnesia aggregate as described above, a degraded layer easily peeled off by a change in temperature is formed on the surface layer of the magnesia aggregate, leading to a lower thermal spalling resistance of the magnesia aggregate.
In order to solve the above-mentioned problems, a magnesia aggregate for a refractory article and a method for manufacturing same are disclosed in Japanese Patent Publication No. 51-30,561, dated Sept. 1, 1976 (hereinafter referred to as the "prior art" ). The method for manufacturing a magnesia aggregate for a refractory article of the prior art comprises the following steps: heating sludgy magnesium hydroxide to dry same; forming said dried magnesium hydroxide into a formed body; provisionally firing said formed body into a formed body comprising magnesia; forming again said formed body comprising magnesia under a high pressure into a green aggregate; and firing said green aggregate.
According to the above-mentioned prior art, in which the green aggregate is formed under a high pressure, the resultant magnesia aggregate for a refractory article has a high density and slag penetration resistance of the aggregate is somewhat improved. However, since the basic structure of the aggregate does not change, a remarkable improvement of slag penetration resistance cannot be expected.
Under such circumstances, there is a strong demand for the development of a magnesia aggregate for a refractory article, which is remarkably excellent in slag penetration resistance, and a method for manufacturing same, but such an aggregate and a manufacturing method thereof have not as yet been proposed.